A variety of kinds of automatic power transmission devices have been developed for the vehicles in order to facilitate the driving of a vehicle and to reduce fatigue to the driver. A representative example may be an automatic transmission comprising a torque converter and a planetary gear mechanism in combination, as is widely used as the power transmission device for the so-called automatic transmission vehicles (AT vehicles). In addition to the automatic transmission, there is a power transmission device by using a transmission of the type of parallel shaft gear mechanism similar to that of a manual transmission vehicle (MT vehicle) in combination with an automatic clutch to shift the transmission steps depending upon the traveling state of the vehicle by using an automatic control device. There is also a power transmission device which shifts the transmission steps responsive to a speed-change instruction from a speed-change lever operated by a driver instead of shifting the transmission steps by using the electronic control device.
A torque converter in the automatic transmission is a transmission device that utilizes a fluid involving, however, a loss of power transmission. Besides, the planetary gear mechanism and a control device, therefore, are complex and expensive. The transmission of the type of parallel shaft gear mechanism involves no transmission loss that stems from the interposition of the torque converter. Therefore, the automatic power transmission device that uses this transmission is superior to the automatic transmission in vehicle fuel efficiency. Besides, the constitution and control of the transmission is simpler and more reliable than those of the automatic transmission.
On the other hand, an automatic speed-changing device that uses the transmission of the type of parallel shaft gear mechanism tends to produce a larger shock of change at the time of shifting the transmission steps than that of the automatic transmission. In the transmission of the type of parallel shaft gear mechanism, a main shaft and a counter shaft arranged in parallel are provided with a plurality of gear trains and a dog clutch, and a speed-changing sleeve of the dog clutch is brought in mesh with one of the gear trains to transmit the engine power to an output shaft. At the time of shifting the transmission step, the speed-changing sleeve that is in mesh is disengaged and is brought into mesh with a new gear train while operating a synchronizer mechanism for attaining synchronism. At this moment, however, the clutch between the engine and the transmission is disconnected, and the transmission of the engine power is interrupted for a short period of time. In the automatic transmission interposing the torque converter, on the other hand, the transmission of power is not substantially interrupted even at the time of shifting the transmission step; i.e., the speed can be smoothly changed with little shock of change.
Concerning the transmission of the type of parallel shaft gear mechanism, there has been known a so-called dual clutch transmission (or a twin clutch transmission) equipped with two clutches and two transmission input shafts connected to the respective clutches in order to continuously transmit the power even at the time of changing the speed while preventing shock as disclosed in, for example, JP-A-8-320054. In the dual clutch transmission as shown in FIG. 5, there are arranged a first input shaft S1 and a second input shaft S2 of a double tubular structure, the second input shaft S2 extending rearward penetrating through the hollow first input shaft S1. A first clutch C1 and a second clutch C2 are provided on the inside and on the outer circumferential side being arranged in concentric in front of the transmission, the clutches being connected on the input side thereof to the engine output shaft. The clutches in this example are wet multi-plate clutches, the output shaft of the first clutch being integrally connected to the hollow first input shaft S1, and the output shaft of the second clutch C2 being integrally connected to the second input shaft S2. Instead of being arranged in concentric, the two clutches may be arranged in parallel in the axial direction. Further, the clutches may be dry single-plate clutches instead of the wet multi-plate clutches.
In the dual clutch transmission, an intermediate shaft (counter shaft) S3 is disposed in parallel with the first input shaft S1 and the second input shaft S2, and an output shaft S4 of the transmission is arranged at the back of the transmission being connected to a propeller shaft of the vehicle. A gear fixed to the rear end of the intermediate shaft S3 comes in mesh with a gear fixed to the output shaft S4 to form an output shaft reduction gear train RG. At the time of changing the speed, the engine power from the first input shaft S1 or the second input shaft S2 is transmitted to the output shaft S4 through the intermediate shaft S3. Through the output shaft reduction gear train RG, the rotational speed of the output shaft S4 becomes lower than the rotational speed of the intermediate shaft S3. Therefore, the transmission is of the so-called output reduction type.
To change the speed, many gear trains having different reduction ratios are arranged among the first input shaft S1, the second input shaft S2 and the intermediate shaft S3. The transmission of this example includes six forward speeds and a reverse gear. The hollow first input shaft S1 has, being fixed thereto, the gears of a second speed gear train G2, a fourth speed gear grain G4 and a sixth speed gear train G6, which are even speeds, while the second input shaft S2 has, being fixed thereto, the gears of a first speed gear train G1 and a third speed gear train G3, which are odd speeds, as well as a reverse gear train GRV. These fixed gears are in mesh with their corresponding loosely fitted gears that are loosely fitted, i.e. rotatably fitted to the intermediate shaft S3. Thus, the gear trains of every other transmission steps are arranged on the input shafts of the dual clutch transmission.
At the rear end of the second input shaft S2, a direct connection clutch C3 of the type of dog clutch is arranged being directly connected to the output shaft S4 to constitute a fifth speed of the transmission. Therefore, the sixth speed constitutes a so-called overtop transmission step of which the output shaft runs faster than the input shaft. The gears loosely fitted to the intermediate shaft S3 have dog teeth (gear splines) formed integrally therewith. On the intermediate shaft S3 are, further, arranged a second speed/fourth speed shifting device X1, a sixth speed/third speed shifting device X2 and a first speed/reverse gear shifting device X3. These shifting devices are constituted as dog clutches as shown in FIG. 3 being provided with a speed-changing sleeve and a synchronizer mechanism as usually used in the transmission of the type of parallel shaft gear mechanism.
In this dual clutch transmission, if the vehicle is traveling at, for example, the second speed, the first clutch C1 is connected, the second clutch C2 is disconnected, the second speed/fourth speed shifting device X1 is in mesh with the second speed gear train G2, and other shifting devices are at their neutral positions. The engine power drives the intermediate shaft S3 through the first input shaft S1 connected to the first clutch C1 and through the second speed gear train G2 and, further, drives the output shaft S4 through the output shaft reduction gear train RG at the rear end. To shift the second speed to the third speed, the first clutch C1 is disconnected, the second speed/fourth speed shifting device X1 is brought to out of mesh and is maintained in a neutral state, the sixth speed/third speed shifting device X2 is brought in mesh with the third speed gear train G3, and the second clutch C2 is connected. After the speed has been shifted, the engine power is transmitted from the second input shaft S2 to the intermediate shaft S3 through the third speed gear train G3, and drives the output shaft S4 through the output shaft reduction gear train RG.
The dual clutch transmission includes the first input shaft S1 and the second input shaft S2 that are arranged being connected to the clutches, respectively, and these input shafts are rotatable independently from each other. At the time of shifting the second speed to the third speed, therefore, it is made possible to bring the sixth speed/third speed shifting device X2 into mesh with the third speed gear train G3 prior to disconnecting the first clutch C1 or prior to bringing the second speed/fourth speed shifting device X1 into out of mesh. Thus, the transmission step can be shifted without substantially interrupting the transmission of the engine power and, therefore, the speed can be changed without shock of change if the second clutch C2 is connected while disconnecting the first clutch C1 after the sixth speed/third speed shifting device X2 has been brought in mesh with the transmission step of the third speed in advance. Further, when the shifting device is to be brought in mesh with the third speed gear train G3, the synchronizer mechanism is operated. Here, however, since an ample surplus time is available for the synchronism, load such as frictional force acting on the synchronizer mechanism can be decreased as compared to that of the synchronizer mechanism in an ordinary transmission of the type of parallel shaft gear mechanism.
If the dual clutch transmission is constituted in the output reduction type, the speed of the output shaft is reduced by the output shaft reduction gear train at the rear end of the intermediate shaft and an increased torque is transmitted. Therefore, the shaft torque of the intermediate shaft can be decreased, the diameter of the shaft and the thickness of the gears (length in the axial direction) can be decreased making it possible to realize the transmission which is light in weight and compact in size. However, the intermediate shaft rotates at a rotational speed higher than that of the output shaft at all times. When the vehicle is traveling at a high speed in which the output shaft rotates at a high speed, therefore, the intermediate shaft rotates at a considerably high speed. A lubricating oil is fed in the housing of the transmission, and the intermediate shaft and gears of the transmission rotate in a state of being dipped in the lubricating oil stored in the bottom portion of the housing; i.e., the lubricating oil is stirred and splashed onto the internal parts of the transmission. Therefore, if the intermediate shaft rotates at high speeds, the lubricating oil is vigorously stirred, the temperature of the lubricating oil rises, the fluid resistance acting on the intermediate shaft increases accompanying the stirring of the lubricating oil, resulting in an increase in the loss of power transmission in the transmission.
The assignment of the present invention is to solve the above problem by preventing the intermediate shaft of the dual clutch transmission from rotating at high speeds when the vehicle is traveling at high speeds.